Combine first 6 ingredients in a large saucepan. Bring to a boil; cover, reduce heat, and simmer 45 minutes. Pour mixture through a wire-mesh strainer into a bowl; discard celery, carrot, peppercorns, and clove. Remove meat from neck; finely chop neck meat and giblets.
Cook onion in butter and oil in a large Dutch oven over medium-high heat, stirring constantly, until onion is lightly browned and tender.
Sprinkle cornstarch over onion and cook, stirring constantly, 2 minutes. Add strained broth. Cook over medium-high heat, stirring constantly with a wire whisk, until thickened and bubbly. Stir in chopped neck meat and giblets and heavy cream.
Skim fat from reserved pan drippings of roasted turkey; discard fat. Stir pan drippings into gravy. Cook over low heat, stirring constantly, until thoroughly heated. Add salt and pepper to taste.